NOTE: I found the 2.5mm is tight for the roller ink I am using. The very tip of the ink fits fine but there is a taper up to about 3mm just below the 5mm shoulder. I ended up drilling a 3mm whole (before the 2.5mm hole) about 5mm deeper than the 5mm hole. If you can follow that it might solve your issue if you find the ink it a bit tight.
NOTE: I found the 2.5mm is tight for the roller ink I am using. The very tip of the ink fits fine but there is a taper up to about 3mm just below the 5mm shoulder. I ended up drilling a 3mm whole (before the 2.5mm hole) about 5mm deeper than the 5mm hole. If you can follow that it might solve your issue if you find the ink it a bit tight.
This is awesome advice!! Thanks so much for always sharing your work and insights!